/**  
 * 文本框根据输入内容自适应高度  
 * @param                {HTMLElement}        输入框元素  
 * @param                {Number}                设置光标与输入框保持的距离(默认0)  
 * @param                {Number}                设置最大高度(可选)  
 */
var autoTextarea = function(elem, extra, maxHeight) {
	extra = extra || 0;
	var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
		isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
		addEvent = function(type, callback) {
			elem.addEventListener ?
				elem.addEventListener(type, callback, false) :
				elem.attachEvent('on' + type, callback);
		},
		getStyle = elem.currentStyle ? function(name) {
			var val = elem.currentStyle[name];
			if(name === 'height' && val.search(/px/i) !== 1) {
				var rect = elem.getBoundingClientRect();
				return rect.bottom - rect.top -
					parseFloat(getStyle('paddingTop')) -
					parseFloat(getStyle('paddingBottom')) + 'px';
			};
			return val;
		} : function(name) {
			return getComputedStyle(elem, null)[name];
		},
		minHeight = parseFloat(getStyle('height'));
	elem.style.resize = 'none';
	var change = function() {
		var scrollTop, height,
			padding = 0,
			style = elem.style;
		if(elem._length === elem.value.length) return;
		elem._length = elem.value.length;
		if(!isFirefox && !isOpera) {
			padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
		};
		scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
		elem.style.height = minHeight + 'px';
		if(elem.scrollHeight > minHeight) {
			if(maxHeight && elem.scrollHeight > maxHeight) {
				height = maxHeight - padding;
				style.overflowY = 'auto';
			} else {
				height = elem.scrollHeight - padding;
				style.overflowY = 'hidden';
			};
			style.height = height + extra + 'px';
			scrollTop += parseInt(style.height) - elem.currHeight;
			document.body.scrollTop = scrollTop;
			document.documentElement.scrollTop = scrollTop;
			elem.currHeight = parseInt(style.height);
		};
	};
	addEvent('propertychange', change);
	addEvent('input', change);
	addEvent('focus', change);
	change();
};

export {
	autoTextarea
}